I have used dates in file names in format 2021_09_30 compared to default in placeholders of 2021-09-30.
I have done a work around using individual placeholders for year_month_day, however it strikes me there must be a more elegant solution, … surely I am not the sole DT users with this legacy file naming format?. 
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This workaround is currently the only possibility.
Thank you for update. I solved my problem by random experimentation.
I changed the default (medium) date in System Preferences and now get an integrated placeholder (rather than three) with my required format. This looks more elegant and takes less space and I don’t see any downside re general date formatting, …
The only disadvantage coming to my mind is that this might be undesired in other apps using the medium format too.